The long-term goal of this project is to understand how cooperation and competition between microbes shapes the composition of human gut microbial communities (microbiota). These dynamics produce interpersonal differences in microbial community composition that far exceed the interpersonal differences in our genomes. This microbial variation is linked to disease predisposition inside and outside of the gut and also to individual response to therapeutics. Although significant progress has been made documenting the diversity of the human gut microbiota, much less is known about the underlying principles that explain how these communities form and change. This lack of knowledge means that we cannot identify which microbial pathways represent appropriate targets for therapeutic intervention, how to shift the balance of species in the gut, or how to successfully introduce new species into these communities. Thus, understanding how metabolites are exchanged between human gut microbes and which of these networks represent the fundamental mechanisms that determine community structure is an important goal. To address this challenge, this grant application describes experiments to dissect the discovery that largely unappreciated small molecules related to vitamin B12 (corrinoids) determine human gut microbial community composition. Aim 1 will examine how the unique features of corrinoid transporters in the human gut microbiome impact the assembly and function of these systems. Aim 2 will test the hypothesis that the greater than 30 different types of corrinoid utilization systems in the human gut microbiome are specialized for acquisition of distinct corrinoids. Aim 3 examines whether different corrinoids (and by extension, the microbes that produce these molecules) induce shared or distinct responses in prominent human symbionts. Completion of these studies will produce a mechanistic understanding of how corrinoids from diet and microbial sources determine community composition in the human gut. Understanding these mechanisms of community interaction provides a basis for therapeutic applications to alter the composition of gut microbial communities.